Dynamic Compressive Behaviors of Two-Layer Graded Aluminum Foams under Blast Loading

Experimental and numerical analyses were carried out to reveal the behaviors of two-layer graded aluminum foam materials for their dynamic compaction under blast loading. Blast experiments were conducted to investigate the deformation and densification wave formation of two-layer graded foams with positive and negative gradients. The shape of the stress waveform changed during the propagation process, and the time of edge rising was extended. Finite element models of two-layer graded aluminum foam were developed using the periodic Voronoi technique. Numerical analysis was performed to simulate deformation, energy absorption, and transmitted impulse of the two-layer graded aluminum foams by the software ABAQUS/Explicit. The deformation patterns were presented to provide insights into the influences of the foam gradient on compaction wave mechanisms. Results showed that the densification wave occurred at the blast end and then gradually propagated to the distal end for the positive gradient; however, compaction waves simultaneously formed in both layers and propagated to the distal end in the same direction for the negative gradient. The energy absorption and impulse transfer were examined to capture the effect of the blast pressure and the material gradient. The greater the foam gradient, the more energy dissipated and the more impulse transmitted. The absorbed energy and transferred impulse are conflicting objectives for the blast resistance capability of aluminum foam materials with different gradient distributions. The results could help in understanding the performance and mechanisms of two-layer graded aluminum foam materials under blast loading and provide a guideline for effective design of energy-absorbing materials and structures.

Fluid–Structure Interaction Effects in the Dynamic Response of Free-Standing Plates to Uniform Shock Loading

The problem of uniform shocks interacting with free-standing plates is studied analytically and numerically for arbitrary shock intensity and plate mass. The analysis is of interest in the design and interpretation of fluid–structure interaction (FSI) experiments in shock tubes. In contrast to previous work corresponding to the case of incident blast profiles of exponential distribution, all asymptotic limits obtained here are exact. The contributions include the extension of Taylor’s FSI analysis for acoustic waves, the exact analysis of the asymptotic limits of very heavy and very light plates for arbitrary shock intensity, and a general formula for the transmitted impulse in the intermediate plate mass range. One of the implications is that the impulse transmitted to the plate can be expressed univocally in terms of a single nondimensional compressible FSI parameter.

The Underwater Blast Resistance of Metallic Sandwich Beams With Prismatic Lattice Cores

The finite element method is used to evaluate the underwater blast resistance of monolithic beams and sandwich beams containing prismatic lattice cores (Y-frame and corrugated core) and an ideal foam core. Calculations are performed on both free-standing and end-clamped beams, and fluid-structure interaction effects are accounted for. It is found that the degree of core compression in the free-standing sandwich beam is sensitive to core strength, yet the transmitted impulse is only mildly sensitive to the type of sandwich core. Clamped sandwich beams significantly outperform clamped monolithic beams of equal mass, particularly for stubby beams. The Fleck and Deshpande analytical model for the blast response of sandwich beams is critically assessed by determining the significance of cross-coupling between the three stages of response: in stage I the front face is accelerated by the fluid up to the point of first cavitation, stage II involves compression of the core until the front and back faces have an equal velocity, and in stage III the sandwich beam arrests by a combination of beam bending and stretching. The sensitivity of the response to the relative magnitude of these time scales is assessed by appropriately chosen numerical simulations. Coupling between stages I and II increases the level of transmitted impulse by the fluid by 20–30% for a wide range of core strengths, for both the free-standing and clamped beams. Consequently, the back face deflection of the clamped sandwich beam exceeds that of the fully decoupled model. For stubby beams with a Y-frame and corrugated core, strong coupling exists between the core compression phase (stage II) and the beam bending/stretching phase (stage III); this coupling is beneficial as it results in a reduced deflection of the back (distal) face. In contrast, the phases of core compression (stage II) and beam bending/stretching (stage III) are decoupled for slender beams. The significance of the relative time scales for the three stages of response of the clamped beams are summarized on a performance map that takes as axes the ratios of the time scales.

III. An automatic method for the investigation of velocity of transmission of excitation in mimosa

The question whether the transmitted effect in Mimosa is due to a hydro mechanical or excitatory impulse is held to be of much interest in Plant Physiology. I shall therefore deal with the subject in some detail and adduce results of additional investigations that I have carried out recently. The general belief that the transmitted impulse in the plant is hydro-mechanical has been largely based on two well-known experiments of Pfeffer and Haberlandt. In the former of these the effect of strong stimulus was found to travel over chloroformed parts of the stem. Pfeffer assumed that the conductivity of this portion must have been abolished, since chloroform is known to abolish motile excitability. In the experiment of Haberlandt, an intervening tissue was killed by scalding; in spite of this, stimulus was found to be transmitted across the scalded area. From these two experiments it was inferred that the impulse which was transmitted could not have been of a true excitatory nature. It was held, on the contrary, that the strong stimulus had given rise to a variation, whether of increase or diminution, of hydrostatic pressure. This variation of pressure, it was assumed, had been hydromechanically transmitted, and, on reaching the distant pulvinus, had inflicted on it a blow which had proved as effective as if a mechanical stimulus had been applied locally. It is thus held that in Mimosa there is a mere transmission of stimulus but no transmission of excitation.